Infrared source

ABSTRACT

STRUCTURE TREATED WITH A COATING AND PITTED WITH AN ABRASIVE THEREBY EMITTING INFRARED SPECTRUM MEETING OR EXCEEDING GLOBAR EMISSIVITIES IN AIR. A METAL ENVELOPE ENCASING A RESISTANCE WIRE WOUND UPON A CORE AND AN INSULATING MATERIAL THEREBETWEEN, AXIAL LEAD OF THE ENDS OF THE RESISTANCE WIRE CONNECTED TO AN EXTERNAL SOURCE OF ELECTRICAL ENERGY, AND THE TEXTURED AND CARBON COATED EXTERNAL SURFACE OF THE SAID METAL ENVELOPE.

H. R. CARLON INFRARED SOURCE Feb. 9, 1971 4 Sheets-Sheet 4 OriginalFiled May 24. 1967 Fig. 5

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me 1 RI o 0 m 0% m United States Patent, Office US. Cl. 117-213 4 ClaimsABSTRACT OF THE DISCLOSURE Structure treated with a coating and pittedwith an abrasive thereby emitting infrared spectrum meeting or exceedingGlobar emissivities in air. A metal envelope encasing a resistance wirewound upon a core and an insulating material therebetween, axial lead ofthe ends of the resistance wire connected to an external source ofelectrical energy, and the textured and carbon coated external surfaceof the said metal envelope.

This is a division of my application Ser. No. 642,662, filed May 24,1967, now Pat. No. 3,450,864.

This invention relates to a new highly emissive source of infraredradiation. More specifically, a standard line of cartridge-heaters canbe modified by sandblasting and carbonoid lacquer coating to yieldhighly emissive infrared radiation sources which will operate withdesirable voltage and current characteristics over a wide range of powerratings.

Another object is the provision of an infrared source permitting AC orDC operation at Globar range of tem peratures, over a wide range ofpower ratings, using con ventionally available voltages at moderatecurrents.

A further object is to provide an infrared source which is rugged,inexpensive, environmentally stable, and longlived. The source iscapable of continuous operation in excess of one year at colortemperatures of about 850 C., without periodic maintenance orsubstantial change in spectral quality.

In accordance with the present invention, a cartridgeheater issandblasted and thickly coated with carbon in a volatile vehicle. Thecoated heater is subjected to operating temperature, thereby an ashresidue is formed on the surface.

At the present time most infrared spectrometers use an infraredradiation source that is either a Globar (sintered silicon carbide) or aNernst Glower (zirconium oxide mixed with yttrium oxides).

In operating an infrared Nornst Glower source, for example, it is normalto use AC operation where possible. Where DC is more convenient, such asin the field, it is necessary to reverse the current at intervals offrom a few seconds to several minutes to prevent deterioration of theGlower. The miniature (0.75 mm. x 0.75 mm.) Nernst Glower (Glower) canbe built to operate with as little as 4 watts of power, and hasconvenient, attached lead, although separate support of the rod is stillrequired. The Glower, however, operates over a restricted temperaturerange (although higher than a standard heater) and requires preheatingto begin operation. The Glower unit is fragile, operable only on AC iflong life is required, has a low thermal mass, and is poor in emissivityin the near and intermediate infrared.

The Glower is brittle and subject to lead open-circuiting under certainoperating conditions, and the leads are simply wrapped on the body ofthe unit and secured with a brittle cement which is easily broken orpulled lose.

The Globar is a rod of bonded silicon carbide. Its usual configurationis about 5 cm. long and 5 mm. in

Patented Feb. 9, 1971 diameter. It is heated electrically from a powersupply of about 180 watts at 50 volts, and has a lifetime of about 250hours when operating at about 1500 K., in air. However, it has beenshown that this source can be operated at temperatures up to 2200 K.Silver electrodes are employed, and the ends of the Globar are silvercoated. In order to prevent overheating of the end connections, a watercooled jacket is utilized as an external cooling means.

The method of treating the surface of the cartridgeheater is bysandblasting and coating the surface with a lacquer containing carbon inthe vehicle.

Sandblasting is one of the simplest and most effective methods ofproducing a scarred or textured surface. The marking of surfaces is bythe discharge of fine abrasives or other grit material through a nozzleat high velocity by means of compressed air. The apparatus and methodused in this invention is similar to that utilized for marking andengraving either glass or metal, for example, hand bellows or powerdriven compressor utilizing a flexible hose carrying the blast nozzlewhich is directed upon the desired surface. The fine abrasives can beeither natural material such as garnet, dolomite, flint quartz andsilica or manufactured materials such as aluminum oxide, silicon andcarbide; the grit can be angular metallic particles such as crushedhardnened steel. Selection of a suitable abrasive in reference to type,size and hardness for a specific application is influenced by the typeof surface to be textured. The abrasive should at all times be movingover the surface in order to have the markings uniform over the surfaceand thus avoiding-the undesired excessive marking at localized points.The silica grit size can be in the range from 20 to 200 mesh and above,and the nozzle can vary from A to /2 inch in diameter, and the pressurecan be in the range from 30 to 90 pounds per square inch.

A lacquer is utilized for coating the surface after sandblasting. Thelacquer comprises carbon in fine subdivision in the proportion of about20-25% in a volatile solvent in the proportion of -75%. Various types ofcarbon may be employed such as channel black, furnace black, thermalblack, acetylene black, and lamp black. The volatile solvent can bearomatics such as toluene, xylene, aliphatic acetates such as ethylacetate, butyl acetate, arnyl acetate, aliphatic alcohols such as ethylalcohol, butyl alcohol, acetone, and methyl-ethyl-ketone.

The carbon material in the volatile solvent is brushed over thesandblasted metal surface in a thick coating to produce total coverageof the surface, and the cartridge heated to operating temperature. Thevolatiles are thus burned off leaving a pink-gray ash which may besmudged with the fingers, but the ash is not readily removed from thecartridge body even when considerable mechanical shock is applied to thecartridge-heater. Dipping is another method for coating, howeverelectrical short circuits could occur between the source body and thesolid wire leads coming from the ends of the source; the short circuitscan be avoided by carefully scraping the dried lacquer from the vicinityof the wire leads.

The cylindrical, indirectly heated surface of the car tridge-heaterpermits treatment of the source to produce any desired emissivitypattern through the choice of various coating materials includingceramics. I have found that the ash produced by the lacquer containingthe carbon provides an enhanced emissivity that may approach unity inthe 10-,u region.

The use of coatings to enhance the emissivity of the Nernst Glower andGlobar is not feasible since these sources would be electricallyshort-circuited by the application of a black carbonoid lacquer coatingin view that the cartridge-heaters by construction are electricallyinsulated from the heating element.

FIG. 4 shows a perspective view of the assembled conventionalcartridge-heater.

FIG. 5 is a longitudinal cross sectional view taken along the line 55 ofFIG. 4.

FIG. 6 is a transverse cross sectional view taken along line 6-6 of FIG.5.

FIG. 7 is similar to FIG. 6 and illustrating the provision of thesandblasted and coating on the surface of the sheath.

In FIGS. 4 through 6, a typical cartridge-heater comprises a resistancewire 6 coiled upon a core 2 with the coiled wire inserted along slot 8.The wire in slot 8 is inserted in opening 3 of core 2 permitting theends of the wound wire to exit at the ends of the core opening 3 in theform of lead pins 7. The casing or sheath 4 enclosing the insulatingmaterial 5 is placed between the helical coil on the core and the casingwhich is sealed by a lava plug 1 which is fired hydrus aluminum silicateplug. FIG. 7 is the specific embodiment of my invention showing thesandblasted and coating 9 on the surface of casing. The insulatingmaterial is any one having high electrical insulating properties at hightemperature and high thermal conductivity such as magnesium oxide. Thecore is usually made of ceramic material fabricated to close tolerances.The casing is a metal which aids in the distribution of the heat createdin the coil evenly throughout the surface of the cartridge-heater. Themetal may be those normally used in manufacturing cartridge-heaters suchas aluminum, brass, stainless steel, iron base alloys and nickel alloys.Both solid nickel leads usually exit from the single end through a lavaend plug; the opposite end is sealed with a welded metal disk. 'I havefound that it is advantageous to modify the heater by having each nickellead exit from the two separate ends through the lava plugs. Since thecartridge is heated internally through a resistance winding, the glowarea of the cartridge may be varied by changing the length and diameterof the winding. Voltage and current characteristics may be varied bychanging the gauge of resistance wire. The commercially availableheaters will operate from 115 or 230 volts A.C. in power ratings from100 to 2000 watts and physical sizes from OD. x 1" long to O.D. X 12"long.

These heaters have been used for many years in applications where highwattage-density electrical heating is required. Although stock heatersare available at ratings down to 55 watts at 115 volts A.C. or DC, noattempt has been made to determine the practical lower limits ofdimension and power requirement for these units, with the specialconsideration of their use as a source of infrared radiation.

This is not surprising since the commercially availablecartridge-heaters, operating at 850 C., have emissivities dropping fromabout 0.6 at 4 micron wavelengths to about 0.3 or less or longerwavelengths near 12 or 14 microns.

A standard cartridge-heater may be operated at actual free-airtemperatures up to 1150 C. for a short-lifetime application, e.g., hoursat power dissipation of 24 watts or more or at temperatures near 800 C.for power inputs near 10 watts, with lifetimes in excess of 5000 hours.Normal operating requirements for a true temperature of 1060" C. are1.07 amperes at 18.7 volts A.C. or DC. for power consumption of wattscontinuously varying about this value. The commercially availablecartridge-heaters may be designed to operate with virtually any commoninput voltage consistent with physical size limitations, except thatvalues below 24 volts are a little more difiicult to achieve.

The more specific commercial available cartridgeheater is cylindrical, 1cm. long x 4 mm. diameter, and consists of a resistance wire wound on aninsulating core, packed in magnesium oxide within a metal tube orsheath. Each solid lead is approximately 5 cm. long. In operation, thebody glows uniformly over its central 7 mm.

length with a 1.5 mm. zone at each end--the depth of the lava endcaps-operating at 50 C. below the temperature of the control zone. Theoverall configuration of the cartridge-heater is similar to a /2 wattelectronic resistor and, in fact, the unit could be soldered unto asupport if so desired.

The untreated cartridge-heater otters distinct advan tages over theconventional Globar and Nornst Glower based on the power consumption andconfiguration. Globar, in the overall power consumption, is greater thanthe cartridge-heater in its compact configuration. The said heater has ahigher voltage and lower current operation thereby permitting a moreefiicient power supply, either from special power pack or from the A.C.line. Since the said heater is electrically connected through two solidloads emerging through a ceramic insulator at one end of the shell,there is no need for the silver solder techniques used with the lNernstGlower, or for the ohmic clip contacts, frequently intermittant, usedwith Globars.

Emissivity measurements of the untreated cartridge heaters, NernstGlower and Globar are shown in FIG. 2. Emissivity is the measure of theradiating capability of an object compared to a perfect radiator orblack body. Values range from near zero for a polished surface to unityfor a black body. The cartridge-heater temperature measurements wereobtained through pyrometric means using a Perkin-Elmer model99-double-pass monochromotor, suitably instrumented and confirmatorydata were obtained from Perkin-Elmer model 137 Infracord and Beckman IR5-A spectrophotometers. In FIG. 2 it will be noted that the untreatedcartridge-heater curve (taken for 850 C.) shows emissivities of about0.8 in the visible wavelengths, but this value decreases gradually withincreasing wavelengths to about 0.3 at 12,14. Globar curve (taken for1100 C.) shows a relatively flat value for emissivities of about 0.8 inthe visible wavelength to 12 The Nernst,Glower curve (taken for 1400 C.)shows emissivities about 0.35 at 4a with a sharp increase to about 0.90at wavelength 8 then a relatively fiat curve thereafter. It is patentthat the emissivity of the untreated cartridge-heater does not approachthe emissivities characteristics of either the Globar or Nernst Glower.

FIG. 1 describes the emissivity and wavelength of the untreatedcartridge-heater as compared with the treated that is sandblasted andsandblasted in combination with the carbon black lacquer. The curvesclearly show increase in the emissivity of the treated heater. It wasunexpected to find fiat emissivity curve of about 0.8 in the visiblewavelength to about 7.5 of the combination of carbon coated andsandblasted heater and, in addition, the unobvious increase ofemissivity of 0.9 and above at about 8.5 to 12 and the approaching unityin the 10 region. Sandblasting, combined with the oxide layer formedwhen the cartridge-heater is heated, can raise the emissivity value toabout 0.5 at 7.5 and above. Best results are obtained when the carbonblack is applied after sandblasting.

The cartridge-heater which is sandblasted and lacquer coated emissivityapproaches that of the Globar and Nernst Glower.

FIG. 3 describes the comparison of emissivity of the cartridge-heaterwhich is carbon-coated and sandblasted and Globar and Nernst Glower. Thecurve of the cartridge-heater equals or exceeds the emissivity at mostwavelengths of Globar and Nernst Glower. Now for the first time there isavailable a source of infrared radiation utilizing a modifiedcartridge-heater. These heaters oifer some distinct advantages over theNernst Glower and Globar. More specifically the heater offersadvantageous power requirements compared to the Globar, in that overallpower consumption is less in this more compact configuration (1 cm. longx 4 mm. diameter). Higher voltage and lower current operation permitsmore efiicient power supply, either from special power packs or from ACline.

The power drain of the cartridge-heater is much less than the standardconfigurations of the other infrared sources, in addition, thecartridge-heater is an AC-DC unit behaving exactly the same for eithervoltage source. The Glower cannot be operated on DC for very longwithout deterioration and, the Globar is susceptible to polarizationeifects. Starting is not a problem with the cartridge-heater, whereasthe Glower must be preheated. Globar requires a longer period of time ascompared with the heating time for the cartridge-heater due to thelatters smaller thermal mass.

This method of modifying cartridge-heaters by sandblasting and carbonoidlacquer coating has unlimited application. This is a modern methoddesigned by the inventor to obviate the disadvantages inherent in thestandard sources for infrared radiation. One of its assets, in additionto its low power requirement, is that it opens many applications which'were not previously possible, such as the direct soldering of theheater onto a printed circuit board.

The nature of this invention has been outlined above, and it will beapparent that certain modifications of the present invention come withinthe province of those skilled in the art; however, it is intended thatall such variations be considered within the scope and spirit of thisinvention.

I claim:

1. A method of treating a heat generating structure for emittinginfrared spectrum comprising: the steps of forcing an abrasive of 20 to200 mesh through a nozzle at a pressure in the range of to 90 pounds persquare inch over a metallic exterior surface, applying a coatingcomposition comprising carbon in a volatile solvent to said texturedsurface and subsequently removing the solvent forming a carbon coating.

2. Method in accordance with claim 1, wherein the coating compositioncomprising 2025% carbon and -75% of volatile solvent.

3. Method in accordance with claim 1 wherein the carbon being channelblack, furnace black, thermal black, acetylene black or lamp black.

4. Method in accordance with claim 1 wherein the volatile solvent beingaromatic, aliphatic acetates, aliphatic alcohols or ketones.

References Cited UNITED STATES PATENTS 3,037,142 5/1962 Griffoul et al.117226 WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R. 117-226; 219553

